We found a pergola design that we loved and copied the design but now that it's (mostly) up we are questioning its stability and safety. The 2x6 beams are notched 3" to fit together, which I have read makes them equivalent to 2x3's. We love this design and want to make it sturdy and safe. We plan to put diagonal supports from the posts to beams, but wanted to seek advice before we proceed. The louver boards (see the second picture, they will be built in each of the 8 openings) have added a load. We want to make this stable.

2 Answers
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Well, it seems totally safe to me, it's not going to fall on you unprovoked. You won't be able to drive a truck into it, but you can sit under it. You can't land an aircraft on it, but it should be able to handle the rain and snow. It's not really load bearing, and it looks beautiful.

However, if you're afraid, grab a Kreg joint kit and screw them together at the notches. Do it from the sides, 1/3rd of the way from the edge to the notch. You can put a dowel in the hole, cut it off, sand it down and you'll barely be able to tell anything's there.

I think you have built a fine looking pergola. Congratulations. I'm quite sure you will be fine once you add diagonal support members from the posts to the joists. You can simply screw them (2X stock) into place in the void side of the 4X4 to 2X6 or get fancy and mortise them. Either way is going to give you the lateral support so the structure will not lean. As you add any panels to the sides, they will also give you some added support. I think you should put your table and chairs under there and enjoy! Good Job.